U.S. patent number 6,562,104 [Application Number 09/810,449] was granted by the patent office on 2003-05-13 for method and system for combusting a fuel.
This patent grant is currently assigned to Praxair Technology, Inc.. Invention is credited to Lawrence E. Bool, III, Hisashi Kobayashi.
United States Patent |
6,562,104 |
Bool, III , et al. |
May 13, 2003 |
Method and system for combusting a fuel
Abstract
A method and system is provided for combusting a fuel having
application to a heat consuming device such as a boiler or furnace
or a reactor. An oxygen-containing stream is introduced into one or
more oxygen transport membranes subjected to a reactive purge or a
sweep gas. The oxygen transport membrane(s) can advantageously be
subjected to a reactive purge or a sweep gas passing in a
cross-flow direction with respect to the membranes to facilitate
separation of the oxygen. In case of a reactive purge, temperature
control of the oxygen transport membrane(s) is effectuated by the
use of a suitable heat sink. Further, the oxygen transport
membranes can be arranged in a row and be connected in series such
that retentate streams of ever lower oxygen concentrations are
passed to successive oxygen transport membranes in the row. The
fuel or sweep gas can be introduced in a direction counter-current
to the bulk flow of the retentate streams.
Inventors: |
Bool, III; Lawrence E.
(Hopewell Junction, NY), Kobayashi; Hisashi (Putnam Valley,
NY) |
Assignee: |
Praxair Technology, Inc.
(Danbury, CT)
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Family
ID: |
24971588 |
Appl.
No.: |
09/810,449 |
Filed: |
March 19, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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739278 |
Dec 19, 2000 |
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Current U.S.
Class: |
95/54; 96/10;
96/4; 96/8; 96/9 |
Current CPC
Class: |
B01D
53/22 (20130101); F23C 6/045 (20130101); F23L
7/007 (20130101); Y02E 20/344 (20130101); F23C
2201/102 (20130101); F23C 2900/06041 (20130101); F23L
2900/07005 (20130101); Y02E 20/34 (20130101) |
Current International
Class: |
B01D
53/22 (20060101); F23L 7/00 (20060101); B01D
053/22 () |
Field of
Search: |
;95/39,45,54,288
;96/4,8,10 |
References Cited
[Referenced By]
U.S. Patent Documents
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5160713 |
November 1992 |
Mazanec et al. |
5516359 |
May 1996 |
Kang et al. |
5562754 |
October 1996 |
Kang et al. |
5565017 |
October 1996 |
Kang et al. |
5820654 |
October 1998 |
Gottzman et al. |
5852925 |
December 1998 |
Prasad et al. |
5888272 |
March 1999 |
Prasad et al. |
5964922 |
October 1999 |
Keskar et al. |
6010614 |
January 2000 |
Keskar et al. |
6077323 |
June 2000 |
Nataraj et al. |
6106591 |
August 2000 |
Keskar et al. |
6114400 |
September 2000 |
Nataraj et al. |
6117210 |
September 2000 |
Prasad et al. |
6139604 |
October 2000 |
Gottzmann et al. |
6139810 |
October 2000 |
Gottzmann et al. |
6149714 |
November 2000 |
Kobayashi |
6296686 |
October 2001 |
Prasad et al. |
6309612 |
October 2001 |
Balachandran et al. |
|
Primary Examiner: Spitzer; Robert H.
Attorney, Agent or Firm: Rosenblum; David M.
Parent Case Text
RELATED APPLICATIONS
This is a continuation-in-part of Ser. No. 09/739,278, filed Dec.
19, 2000, now abandoned.
Claims
We claim:
1. A method of combusting a fuel comprising: introducing an
oxygen-containing stream into at least one oxygen transport
membrane projecting into a combustion zone to separate oxygen from
the oxygen-containing stream and thereby, to introduce an oxygen
permeate into the combustion zone; introducing a fuel stream into
the combustion zone; combusting fuel within said fuel stream in the
presence of said oxygen permeate so that said at least one oxygen
transport membrane is subjected to a reactive purge and a portion
of heat arising from the combustion of the fuel heats said at least
one oxygen transport membrane to an operational temperature; and
absorbing radiant heat energy emanating from said at least one
oxygen transport membrane within a heat sink to promote
stabilization of the operational temperature thereof.
2. The method of claim 1, wherein: said at least one oxygen
transport membrane comprises at least one row of oxygen transport
membranes spaced apart from one another; and said fuel stream is
introduced in a cross-flow relationship to said at least one row of
oxygen transport membranes.
3. The method of claim 2, wherein said heat sink comprises tubes of
flowing heat absorbing fluid interspersed within said at least one
row of oxygen transport membranes.
4. The method of claim 3, wherein said tubes of flowing heat
absorbing fluid are steam tubes to heat water flowing therein.
5. The method of claim 3 or claim 4, further comprising: connecting
said at least one row of oxygen transport membranes in series to
produce a flow path of retentate streams passing to successive
oxygen transport membranes having ever more lean oxygen
concentrations; and introducing the fuel stream into said
combustion zone in a counter-current flow direction as viewed with
respect to the flow path of the retentate streams so that said
reactive purge acts in said counter-current flow direction.
6. A method of combusting fuel comprising: introducing an
oxygen-containing stream into at least one oxygen transport
membrane projecting into a separation zone to separate oxygen from
said oxygen-containing stream and thereby, to introduce an oxygen
permeate into said separation zone; heating the at least one oxygen
transport membrane to an operational temperature; introducing a
fuel stream into a combustion zone to combust and thereby to
produce a flue gas stream; circulating a sweep gas stream, composed
of part of said flue gas stream, within the separation zone; and
circulating said sweep gas stream from said separation zone to said
combustion zone to support combustion of said fuel stream.
7. The method of claim 6, wherein: said at least one oxygen
transport membrane comprises at least one row of oxygen transport
membranes spaced apart from one another; and said sweep gas stream
is introduced in a cross-flow relationship to said at least one row
of oxygen transport membranes.
8. The method of claim 7, wherein: the oxygen transport membranes
are connected in series to produce a flow path of retentate streams
passing to successive oxygen transport membranes and having ever
more lean oxygen concentrations; and the sweep gas stream is
circulated in a counter-current flow direction as viewed with
respect to the flow path of the retentate streams.
9. The method of claim 7 or claim 8, wherein said oxygen transport
membranes are heated to said operational temperature by said sweep
gas stream.
10. The method of claim 9, wherein said sweep gas stream is
circulated by cooling a remaining part of said flue gas stream and
injecting said remaining part of said flue gas stream into said
separation zone in the form of at least one jet.
11. The method of claim 9, wherein said sweep gas stream is
circulated by cooling said sweep gas stream after passage through
said separation zone and injecting said sweep gas stream into said
combustion zone by a blower.
12. The method of claim 1 or claim 6, wherein: said oxygen-enriched
stream is air; separation of the oxygen from the oxygen-enriched
stream produces a nitrogen-enriched stream; and said nitrogen
enriched stream is extracted as a product stream.
13. An oxygen enhanced combustion system comprising: at least one
oxygen transport membrane located within a combustion zone to
separate said oxygen from an oxygen-containing stream introduced
into said at least one oxygen transport membrane, thereby to
produce an oxygen permeate; at least one fuel nozzle for injecting
a fuel stream of the fuel into said combustion zone so that said at
least one oxygen transport membrane is subjected to a reactive
purge produced by combustion of the fuel in the presence of said
oxygen permeate and a portion of heat arising from the combustion
of the fuel heats said at least one oxygen transport membrane to an
operational temperature; and a heat sink positioned to absorb
radiant heat energy emanating from said at least one oxygen
transport membrane to promote stabilization of the operational
temperature thereof.
14. The system of claim 13, wherein: said at least one oxygen
transport membrane comprises at least one row of oxygen transport
membranes spaced apart from one another; and said heat sink
comprises tubes of flowing heat absorbing fluid interspersed within
said at least one row of oxygen transport membranes.
15. The system of claim 14, wherein said tubes of flowing heat
absorbing fluid are steam tubes to heat water flowing therein.
16. The system of claim 14 or claim 15, further comprising: said at
least one row of oxygen transport membranes connected in series to
produce a flow path of retentate streams passing to successive
oxygen transport membranes having ever more lean oxygen
concentrations; and said at least one fuel nozzle is positioned to
introduce the fuel stream into said combustion zone in a
counter-current flow direction as viewed with respect to the flow
path of the retentate streams so that said reactive purge acts in
said counter-current flow direction.
17. An oxygen enhanced combustion system comprising: at least one
oxygen transport membrane located within a separation zone to
separate said oxygen from an oxygen-containing stream introduced
into said at least one oxygen transport membrane; at least one
nozzle for injecting a fuel stream into a combustion zone for
combustion of said fuel to produce a flue gas stream; means for
heating said at least one oxygen transport membrane; and means for
circulating a sweep gas stream composed of a part of said flue gas
stream into said separation zone and from said separation zone to
said combustion zone to support combustion of said fuel stream.
18. The system of claim 17, wherein: said at least one oxygen
transport membrane comprises at least one row of oxygen transport
membranes spaced apart from one another; and said sweep gas stream
circulating means circulate said sweep gas stream in a cross-flow
relationship to said at least one row of oxygen transport
membranes.
19. The system of claim 18, wherein: said oxygen transport
membranes are connected in series to produce a flow path of
retentate streams passing to successive oxygen transport membranes
and having ever more lean oxygen concentrations; and the sweep gas
stream is circulated in a counter-current flow direction as viewed
with respect to the flow path of the retentate streams.
20. The system of claim 18 or claim 19, wherein said heating means
comprises heat transfer from said sweep gas stream.
21. The system of claim 20 wherein said sweep gas stream
circulating means comprise: a heat exchanger to cool a remaining
part of said flue gas stream; at least one flue gas nozzle to
inject at least one flue gas jet composed of said flue gas stream;
and a blower interposed between said heat exchanger and said at
least one flue gas nozzle.
22. The system of claim 20, wherein said sweep gas stream
circulating means comprise: a heat exchanger to cool the sweep gas
stream, the heat exchanger positioned to receive said sweep gas
stream after having passed through said separation zone; an inlet
to said combustion zone; and a blower interposed between said heat
exchanger and said inlet to inject said sweep gas stream into said
combustion zone.
Description
FIELD OF THE INVENTION
The present invention relates to a method and system for combusting
fuel that has direct application to heat consuming devices such as
boilers and furnaces as well as reactors that utilize separated
oxygen. More particularly, the present invention relates to such a
combustion method and system in which combustion is enhanced with
oxygen produced by the use of a ceramic membrane system. Even more
particularly, the present invention relates to such a method and
system in which the ceramic membrane system is subjected to a
countercurrent reactive purge or flow of sweep gas.
BACKGROUND OF THE INVENTION
Growing concerns about environmental issues, such as global warming
and pollutant emissions, are driving industries to explore new ways
to increase efficiency and reduce emissions of pollutants. This is
particularly true for fossil fuel fired combustion systems, which
represent one of the largest sources of carbon dioxide and air
pollution emissions. One effective way to reduce emissions and to
increase efficiency is to use oxygen, or oxygen enriched air, in
the combustion process. The use of oxygen or oxygen enriched air
reduces stack heat losses, which increases the system efficiency,
while at the same time reducing NOx emissions. Further, the
concentration of carbon dioxide in the flue gas is higher since
there is little or no nitrogen to act as a diluent. The higher
carbon dioxide concentration enhances carbon dioxide recovery
options.
Oxygen using the prior art has been limited to those processes with
high exhaust temperatures, such as glass furnaces. In such
applications, the fuel savings and other benefits achieved are
greater than the cost of the oxygen. In low exhaust temperature
systems, such as boilers, the reverse is true. In these systems,
the cost of oxygen produced with current technologies is more
expensive than the available fuel savings. This makes oxygen use in
such systems economically unattractive. Moreover, when the energy
required to produce the oxygen is taken into consideration, the
overall thermal efficiency decreases.
Oxygen transport membranes have been advantageously utilized in the
prior art to produce oxygen for heat consuming devices and
processes in a manner that results in a savings of energy that
would otherwise have to be expended in the separation of oxygen.
Oxygen transport membranes are fabricated from oxygen-selective,
ion transport ceramics in the form of tubes or plates that are in
themselves impervious to the flow of oxygen. Such ceramics,
however, exhibit infinite oxygen selectivity at high temperatures
by transporting oxygen ions through the membrane. In oxygen
transport membranes, the oxygen is ionized on one surface of the
membrane to form oxygen ions that are transported through the
membrane. The oxygen ions on the opposite side of the membrane
recombine to form oxygen with the production of electrons.
Depending upon the type of ceramic, oxygen ions either flow through
the membrane to ionize the oxygen or along separate electrical
pathways within the membrane, or by an applied electric potential.
Such solid electrolyte membranes are made from inorganic oxides,
typified by calcium-or yttrium-stabilized zirconium and analogous
oxides having fluoride or perovskite structures.
In U.S. Pat. No. 5,888,272 oxygen transport membranes are
integrated into a combustion process itself, with all the oxygen
produced going directly into the combustor. The heated flue gases
can then be routed to a heat consuming process. In one embodiment,
flue gases are recycled through a bank of oxygen transport membrane
tubes and enriched with oxygen. Typically the flue gas enters the
bank containing anywhere from 1 to about 3 percent oxygen and
leaves the bank containing from about 10 to about 30 percent oxygen
by volume. The enriched flue gas is then sent to a combustion space
where it is used to burn fuel. In another embodiment, called
reactive purge, the oxygen transport membrane tubes are placed
directly in the combustion space. A fuel and flue gas mixture, is
passed through the tubes and combust with the oxygen as it passes
through the tubes. Thus oxygen production and combustion take place
simultaneously inside the oxygen transport membrane with the fuel
diluted with flue gas.
As will be discussed, the present invention utilizes oxygen
transport membranes to produce oxygen to support combustion that
inherently reduces the energy expenditures involved in compressing
an incoming oxygen containing feed to the membranes. Combustion can
take place at the surface of the oxygen transport membranes in the
presence of fuel that is not diluted with flue gas.
SUMMARY OF THE INVENTION
The present invention provides methods and systems for combusting
fuel that have direct application to such heat consuming devices as
boilers and furnaces or to reactors that separate oxygen from an
oxygen-containing feed. Such reactors include devices for
separating oxygen to produce a nitrogen-enriched product.
In accordance with one method of the present invention, an
oxygen-containing stream is introduced into at least one oxygen
transport membrane. The membrane projects into a combustion zone to
separate oxygen from the oxygen-containing stream and thereby, to
introduce an oxygen permeate into the combustion zone. A fuel
stream is introduced into the combustion zone and fuel within the
fuel stream is combusted in the presence of the oxygen permeate so
that the at least one oxygen transport membrane is subjected to a
reactive purge and a portion of heat arising from the combustion of
the fuel heats the at least one ceramic membrane to an operational
temperature. Radiant heat energy emanating from the at least one
oxygen transport membrane is absorbed within a heat sink to promote
stabilization of the operational temperature of the at least one
oxygen transport membrane.
The at least one oxygen transport membrane can comprise at least
one row of oxygen transport membranes spaced apart from one
another. The fuel stream is introduced in a cross-flow relationship
to the at least one row of oxygen transport membranes.
It is to be noted that the term, "cross-flow" as used herein and in
the claims means a flow direction with respect to the oxygen
transport membranes that is at right angles to the length of the
oxygen transport membranes plus or minus about forty-five degrees.
For instance, if tubular oxygen transport membranes are used, the
"cross-flow" direction would be at or near right angles to the tube
as opposed to a direction parallel to the length of the tube as
measured between its ends. As such, in "cross-flow" the fuel stream
and therefore, the reactive purge, can be directed anywhere from an
angle directly in line with the row to a direction at right angles
to the row. Furthermore, the term "row" as used herein and the
claims means any arrangement of oxygen transport membranes in a
single file. The oxygen transport membranes to be in a "row" do not
necessarily, however, have to be positioned so that one oxygen
transport membrane is directly in front of or behind another oxygen
transport membrane. For instance, oxygen transport membranes may be
staggered so that each membrane has full benefit of the reactive
purge, or as will be discussed, a sweep gas such that each oxygen
transport membrane can take full advantage of such a reactive purge
or sweep gas acting at least substantially parallel to the line of
oxygen transport membranes making up a row.
It should be pointed out that a cross-flow arrangement is
advantageous over flow arrangements that act parallel to the length
of the oxygen transport membranes. One major advantage is that all
adjacent oxygen transport membranes, as viewed in a transverse
direction to the reactive purge will see the same combustion
conditions. Furthermore, the fuel composition will be substantially
the same from the top to the bottom of an oxygen transport
membrane. This will promote uniformity in the oxygen flux and
therefore, the combustion flux for the reactive purge along the
length of an oxygen transport membrane. Since, the composition of
the surrounding gas will change as one moves from such transverse
sets of oxygen transport membranes it is conceivable that different
materials could be advantageously used in subsequent sets of oxygen
transport membranes. Furthermore, the rows might be designed to
provide additional transverse sets of such adjacent oxygen
transport membranes that would provide a back-up upon the
degradation of a preceding transverse set of oxygen transport
membranes.
The heat sink with respect to the at least one row of oxygen
transport membranes can comprise tubes of flowing heat absorbing
fluid interspersed within the at least one row of oxygen transport
membranes. The tubes of flowing heat absorbing fluid can be steam
tubes to heat water flowing therein. In such case, the method of
the present invention would be applied to a boiler.
The at least one row of oxygen transport membranes can be connected
in series to produce a flow path of retentate streams passing to
successive oxygen transport membranes having ever more lean oxygen
concentrations. The fuel stream can be introduced into the
combustion zone in a counter-current flow direction as viewed with
respect to the flow path of the retentate streams so that the
reactive purge acts in the counter-current flow direction.
In accordance with another method of the present invention, at
least one oxygen transport membrane projects into a separation zone
to separate oxygen from the oxygen-containing stream and thereby,
to introduce the oxygen permeate into the separation zone. The at
least one oxygen transport membrane is heated to an operational
temperature. A fuel stream is combusted in a combustion zone
located within the heat consuming device to produce a flue gas
stream. A sweep gas stream composed of part of the flue gas stream
is circulated within the separation zone. Further, the sweep gas
stream is circulated from the separation zone to the combustion
zone to support combustion of the fuel stream.
The at least one oxygen transport membrane can comprise at least
one row of oxygen transport membranes spaced apart from one another
and the sweep gas stream can be introduced in a cross-flow
relationship to the at least one row of oxygen transport membranes.
The oxygen transport membranes can be connected in series to
produce a flow path of retentate streams passing to successive
oxygen transport membranes and having ever more lean oxygen
concentrations. In such case, the sweep gas stream can be
circulated in a counter-current flow direction as viewed with
respect to the flow path of the retentate streams. The oxygen
transport membranes can be heated to the operational temperature by
the sweep gas stream.
Advantageously, the sweep gas stream can be circulated by cooling a
remaining part of the flue gas stream and injecting the remaining
part of the flue gas stream into the separation zone in the form of
at least one jet. Alternatively, the sweep gas stream can be
circulated by cooling the sweep gas stream after passage through
the separation zone and injecting the sweep gas stream into the
combustion zone by a blower.
The foregoing method could be used to separate oxygen from air. In
such case, the oxygen-enriched stream is air and separation of the
oxygen from the oxygen-enriched stream produces a nitrogen-enriched
stream. The nitrogen enriched stream can be extracted as a product
stream.
The present invention also provides oxygen-enhanced combustion
systems that again have principal applications to heat consuming
devices and various types of reactors. In one such system, at least
one oxygen transport membrane is located within a combustion zone
to separate oxygen from an oxygen-containing stream introduced into
the at least one oxygen transport membrane, thereby to produce an
oxygen permeate. At least one fuel nozzle is provided for injecting
a fuel stream of the fuel into the combustion zone so that the at
least one oxygen transport membrane is subjected to a reactive
purge produced by combustion of the fuel in the presence of the
permeated oxygen and a portion of heat arising from the combustion
of the fuel heats the at least one ceramic membrane to an
operational temperature. A heat sink is positioned to absorb
radiant heat energy emanating from the at least one oxygen
transport membrane to promote stabilization of the operational
temperature thereof.
The at least one oxygen transport membrane can comprise at least
one row of oxygen transport membranes spaced apart from one
another. The heat sink can comprise tubes of flowing heat absorbing
fluid interspersed within the at least one row of oxygen transport
membranes. The tubes of flowing heat absorbing fluid can be steam
tubes to heat water flowing therein. In such case, the heat
consuming device to which the present invention would be applied
could be a boiler.
The at least one row of oxygen transport membranes can be connected
in series to produce a flow path of retentate streams passing to
successive oxygen transport membranes having ever more lean oxygen
concentrations. The at least one fuel nozzle can be positioned to
introduce the fuel stream into the combustion zone in a
counter-current flow direction as viewed with respect to the flow
path of the retentate streams so that the reactive purge acts in
the counter-current flow direction.
In an alternative system in accordance with the present invention,
at least one oxygen transport membrane is positioned within a
separation zone of the heat consuming device to introduce the
permeated oxygen into the separation zone. At least one nozzle is
provided for injecting a fuel stream into a combustion zone for
combustion of the fuel stream to produce a flue gas stream. A means
is provided for heating the at least one oxygen transport membrane
to an operational temperature. A means is also provided for
circulating a sweep gas stream composed of a part of the flue gas
stream into the separation zone and from the separation zone to the
combustion zone to support combustion of the fuel stream. As in
other embodiments, the at least one oxygen transport membrane can
comprise at least one row of oxygen transport membranes spaced
apart from one another. The sweep gas circulation means circulate
the sweep gas stream in a cross-flow relationship to the at least
one row of oxygen transport membranes. The oxygen transport
membranes can be connected in series to produce a flow path of
retentate streams passing to successive oxygen transport membranes
and having ever more lean oxygen concentrations. In such case, the
sweep gas stream is circulated in a counter-current flow direction
as viewed with respect to the flow path of the retentate streams.
The heating means can comprise heat transfer from the sweep gas
stream to the oxygen transport membranes. The foregoing aspects of
the present invention could be applied to a furnace or a
boiler.
The circulation means can include a heat exchanger to cool a
remaining part of the flue gas stream. Additionally, at least one
flue gas nozzle is provided to inject at least one flue gas jet
composed of the flue gas stream into the separation zone and a
blower interposed between the heat exchanger and the at least one
flue gas nozzle. Alternatively, the circulation means can comprise
a heat exchanger to cool the sweep gas stream. The heat exchanger
is positioned to receive the sweep gas stream after having passed
through the separation zone. An inlet to the combustion zone is
provided and a blower is interposed between the heat exchanger and
the inlet to inject the sweep gas stream into the combustion
zone.
In embodiments of the present invention in which the oxygen
transport membranes are connected in series, as retentate streams
emanating from the oxygen transport membranes are sequentially
introduced into the membranes of the row, the oxygen content of the
feed to each membrane decreases and therefore the amount of oxygen
permeated through each successive membrane also decreases. Thus,
the permeated oxygen in the vicinity of the last of the oxygen
transport membranes in the row is at a lower concentration and
therefore, a lower oxygen partial pressure than at the first of the
oxygen transport membranes in the row. At the same time, the oxygen
partial pressure within each of the oxygen transport membranes is
also successively decreasing as it passes to successive membranes
in a row. If the partial pressure of the permeated oxygen remains
constant or in fact decreases in the vicinity of successive
membranes, the pressure driving force for effecting the separation
in such successive oxygen transport membranes is also
decreasing.
As a result of the ever decreasing pressure driving force, in
successive oxygen transport membranes, in order to effect the
separation at the last oxygen transport membrane in the row, the
separation needs more facilitation by the reactive purge or sweep
gas than at the first of the oxygen transport membranes. This
naturally occurs in the present invention due to the countercurrent
flow of the fuel stream that can act as a reactive purge or the
sweep gas. In case of a reactive purge provided by the fuel stream,
as the fuel flows in the counter-current direction, the fuel is
consumed and thus, the concentration of fuel within the bulk flow
of fuel and combustion gases decreases. As a result, it becomes
increasingly difficult for the fuel to diffuse to the surface of
the membrane and combust. Therefore, the reactive purge is most
effective at the last of the oxygen transport membranes in the row
where the greatest facilitation of oxygen separation by the
reactive purge is required. As the flow of fuel containing gases
flows along the row, diffusion of the fuel to the surface of the
membrane is more difficult due to the dilution of fuel within the
combustion gases. However, less facilitation is required due to the
increasing pressure driving force in successive oxygen transport
membranes towards the first of the oxygen transport membranes.
The action of a counter-current flow of sweep gas has a similar
effect to the reactive purge in that as it flows in the
counter-current direction, it has the lowest concentration of
oxygen at the last of the oxygen transport membranes in the row and
therefore is most able to facilitate the separation at such oxygen
transport membrane. As it travels in the counter-current direction
and gains oxygen, it is least able to facilitate the separation.
However, less facilitation is required in successive oxygen
transport membranes taken in a direction from the last of the
oxygen transport membranes to the first of the oxygen transport
membranes.
As may be appreciated, the use of any reactive purge reduces the
degree of compression for the incoming feed such that only a blower
or an induced draft fan might be necessary to circulate the
oxygen-containing gas into the oxygen transport membranes. The use
of a counter-current reactive purge or sweep gas, reduces the
degree of compression that would otherwise be required to compress
the feed to an oxygen transport membrane system. This reduction of
compressive effort makes the application of the present invention
attractive even in low exhaust temperature systems such as
boilers.
In the present invention, the reactive purge involves the
combustion of fuel in the presence of oxygen separated by the
membrane. As a result, this combustion of oxygen takes place at or
near the surface of the membrane to produce a driving force for the
separation to also lessen or possibly eliminate the degree to which
the incoming oxygen containing feed need be compressed. Hence, the
reactive purge of the present invention has application to any
membrane system whether or not there are multiple membranes used or
multiple membranes are connected in series.
Since, the adiabatic flame temperature of ambient temperature
methane and pure oxygen exceeds 5000.degree. F., direct combustion
of natural gas on the surface of an oxygen transport membrane is
not normally considered. In the prior art, the excessive
temperature problem involved in reactive purging is overcome by
mixing a small amount of fuel with a large amount of non-reactive
purge gas. In many membrane types, the flux of oxygen through the
membrane increases as the membrane temperature increases. The
combustion reaction at the surface, and therefore the heat release
at the surface, is therefore limited by the oxygen flux through the
membrane. However, poor temperature control can lead to
catastrophic thermal runaway of the membrane. As the temperature
increases more oxygen passes through the membrane leading to higher
combustion rates at the surface and still higher membrane
temperatures until the temperature limitations of the membrane is
exceeded. As will be discussed in more detail, the inventors herein
have found that temperature control of the membranes can be
accomplished by appropriate placement or arrangement of the
membranes with respect to a heat sink that can absorb radiant heat
and therefore prevent damaging thermal runaway.
BRIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims distinctly pointing
out the subject matter that Applicants regard as their invention,
it is believed that the invention would be better understood when
taken in connection with the accompanied drawings in which:
FIG. 1 is a fragmentary view of a boiler employing a combustion
method and system in accordance with the present invention;
FIG. 2 is a furnace employing a combustion method and system in
accordance with the present invention;
FIG. 3 is an alternative embodiment of a boiler employing a
combustion method and system in accordance with the present
invention;
FIG. 4 is a graphical illustration of an example of permissible
surface area ratios of oxygen transport membranes and steam tubes
to control membrane temperature within a boiler; and
FIG. 5 is a graphical illustration of an example of permissible
oxygen transport membrane spacing to control membrane temperature
within a glass furnace.
DETAILED DESCRIPTION
With reference to FIG. 1 a boiler 1 in accordance with the present
invention is illustrated. Boiler 1 heats water or steam that is
introduced through an inlet manifold 10 into steam tubes 12, 14, 16
and 18. Either steam or superheated steam is raised within steam
tubes 12, 14, 16 and 18 with the combustion of fuel, for instance,
methane or natural gas, which enters boiler 1 through a fuel inlet
nozzle 20. The steam or superheated steam is discharged through an
outlet manifold 22.
A row of oxygen transport membranes 24, 26, 28 and 30 project into
a combustion zone 31 provided within boiler 1. Compressed air
stream 32 is introduced into a first of the oxygen transport
membranes 24. Oxygen permeates through such first oxygen transport
membrane 24 to produce a retentate stream 34 that has lower
concentration of oxygen than the incoming oxygen-containing stream.
Retentate stream 34 is then introduced into a successive oxygen
transport membrane 26 where still more oxygen is permeated through
the membrane to produce another retentate stream 36 which has a
still lower concentration of oxygen than retentate stream 34.
Retentate stream 36 is introduced into succeeding oxygen transport
membrane 28 in which more oxygen is permeated to produce a
retentate stream 38 which contains a still lower concentration of
oxygen. Retentate stream 38 is introduced into a last of the oxygen
transport membranes 30 in which oxygen further permeates to produce
a retentate stream which is very lean in oxygen as compared to the
incoming air and thus can be taken as a nitrogen product stream
40.
In order to effectuate the foregoing operation, oxygen transport
membranes 24, 26, 28, and 30 are connected in series by an
arrangement of lance tubes 42, 44, 46 and 48. Lance tube 42 is
connected to a source of the air feed to receive air stream 32.
Lance tubes 44, 46, and 48 are connected to the oxygen transport
membranes 26, 28, and 30 so that lance tube 44 is fed with
retentate produced within oxygen transport membrane 24, lance tube
46 is fed with retentate produced within oxygen transport membrane
26, and lance tube 48 is fed with retentate produced within oxygen
transport membrane 28.
As illustrated, fuel is introduced into boiler 1 in a direction
indicated by arrowhead "A" that is counter-current to the flow path
of retentate streams 34, 36, and 38. Thus, at the last of the
oxygen transport membranes 30, fuel rich combustion conditions are
obtained that consume substantially all the permeated oxygen. As
the fuel stream passes through the row of oxygen transport
membranes 24, 26, 28 and 30, the fuel is successively diluted with
combustion products. The combustion of the fuel acts as a reactive
purge that acts in such counter-current flow direction "A". Since,
the reactive purge has the highest fuel concentration at oxygen
transport membrane 30, it is more able to facilitate the separation
of oxygen than at oxygen transport membrane 24. However, since the
separation driving forces (excluding the reactive purge) are
greater at oxygen transport membrane 24 than oxygen transport
membrane 30, less facilitation is required.
The resultant flue gas produced by combustion of the fuel can
either be discharged from combustion zone 31 without further use or
can be employed in a superheat exchanger to form superheated
steam.
It is contemplated that air stream 32 (as well as the incoming air
feeds to the other specifically described embodiments of the
present invention) is not compressed by an external compressor.
Although not illustrated, a blower or an induced draft fan would be
used to overcome flow losses and thereby circulate the air or other
oxygen-containing gases to the oxygen transport membranes.
Embodiments of the present invention are, however, possible in
which the incoming feed is compressed. As is known to those skilled
in the art the degree of compression will depend on the degree of
oxygen separation required and the additional oxygen separation
driving forces provided by the reactive purge or in other
embodiments, the sweep gases used.
With reference to FIG. 2, a furnace 2 is illustrated in which fuel
is injected via a nozzle 50 into a combustion zone 52 to be
combusted and thereby produce heat for heating a heat load such as
a melt. The combustion of the fuel is supported by oxygen produced
by oxygen transport membranes 54, 56, 58 and 60 that project into a
separation zone 61 separated from combustion zone 52 by means of a
baffle plate 62.
Oxygen transport membranes 54, 56, 58, and 60 function in a similar
manner to oxygen transport membranes 24, 26, 28 and 30 of boiler 1.
In this regard, oxygen transport membranes 54, 56, 58 and 60 are
connected in series and are fed by an air stream 64 to produce
retentate streams 66, 68, 70, and 72 having an ever decreasing
oxygen concentration. Retentate stream 72 can be taken as a
nitrogen product stream. The series connection between oxygen
transport membranes 54, 56, 58, and 60 is effectuated by lance
tubes 74, 76, 78, and 80. Lance tube 74 receives compressed air
stream 64, lance tube 76 receives retentate stream 66, lance tube
78 receives retentate stream 68, and lance tube 80 receives
retentate stream 70.
As a result of the combustion of the fuel within combustion zone
52, a heated flue gas stream 81 is provided. Part of the heated
flue gas stream 81 is used to form a sweep gas stream 82 that is
circulated into separation zone 61 in a countercurrent flow
direction to the flow of compressed air stream 64 and retentate
streams 66, 68, 70, and 72 within oxygen transport membranes 54,
56, 58 and 60. As sweep gas stream 82 travels in the
counter-current flow direction, it gains more oxygen and is
circulated back to combustion zone 52 as an oxygen-enriched flue
gas stream 83 to support the combustion.
As stated above, since oxygen transport membranes 54, 56, 58 and 60
are connected in series, the oxygen content within retentate
streams 66, 68 and 70 steadily decreases as does the amount of
oxygen permeated through successive oxygen transport membranes. For
instance, less oxygen permeates through oxygen transport membrane
58 than oxygen transport membrane 56. As sweep gas stream 82
encounters a last oxygen transport membrane 60 within the row, it
has the least concentration of oxygen to most facilitate the
permeation of oxygen through oxygen transport membrane 60. As sweep
gas stream 82 flows towards oxygen transport membrane 54 (the first
in the row), it gathers more oxygen and is therefore less able to
facilitate the permeation of oxygen. However, since the oxygen
concentration of the feed to oxygen transport membrane 54 is
greater than at successive oxygen transport membranes, less
facilitation is required.
The circulation of sweep gas stream 82 may be accomplished by means
of the motive force of the fuel stream, and the propagation of
combustion taking place in combustion zone 52 in the flow direction
of air stream 64 and retentate streams 66, 68, 70 and 72. Furnace 2
utilizes more elaborate means. In furnace 2, the circulation is
aided by dividing heated flue gas stream 81 into remaining parts 84
and 85. Remaining part 85 can be discharged. Remaining part 84 is
further cooled in a heat exchanger 86 to a temperature low enough
for effective use of a blower, yet above the water dew point in
remaining part 85. Heat exchanger 86 contains a tube bundle
oriented in a cross-flow direction and filled with circulating heat
transfer media, for instance, water, steam or air. The resultant
cooled flue gas stream produced from remaining part 84 is
introduced into a blower 87 that is connected to flue gas nozzles
88 to inject flue gas jets in the countercurrent direction into the
row of oxygen transport membranes 54, 56, 58 and 60. The flue gas
jets create more sweep gas and help circulation of sweep gas stream
82 within furnace 2. As may be appreciated, although eight flue gas
nozzles 88 are shown, in a possible embodiment of the present
invention only a single flue gas nozzle and therefore a single flue
gas jet might be required for the particular circulation
requirements.
Oxygen transport membranes 54, 56, 58 and 60 are heated to
operational temperature by sweep gas stream 82. In practice, sweep
gas stream 82 being formed from a portion 89 of heated flue gas
stream 81 is potentially at a temperature that is well in excess of
the operational temperature of oxygen transport membranes 54, 56,
58 and 60. Sweep gas stream 82 is, however, sufficiently cooled by
entrainment in flue gas jets that are formed from remaining part 84
of heated flue gas stream 81 that has been cooled within heat
exchanger 86.
The furnace exit gas temperature and the desired operating
temperature of the oxygen transport membranes 54, 56, 58 and 60
define the optimal ratio of the cooled flue gas to hot flue gas,
namely, the ratio between remaining part 84 and portion 89 of
heated flue gas stream 81. The calculations to determine this are
based on a simple mass and energy balance. For example, assuming
remaining part 84 of heated flue gas stream 81 has been cooled to
about 400.degree. F., an operational temperature of oxygen
transport membranes 54, 56, 58 and 60 of about 1800.degree. F., for
a furnace exit gas temperature of about 2100.degree. F., about 20%
of sweep gas stream 82 should be made up of remaining part 84 of
heated flue gas stream 81, after having been cooled.
As may be appreciated, part or all of the make-up for the jets
emanating from flue gas nozzles 88 could be steam.
With additional reference to FIG. 3, a boiler 3 is illustrated in
which a fuel stream is introduced into a combustion zone 90 by way
of a fuel nozzle 92. Combustion of the fuel stream produces heat
that is used to boil water or superheat steam within steam tubes
94.
A flue gas stream 96 produced by the combustion is separated so
that a part 97 thereof is introduced into a separation zone 98 as a
sweep gas. A remaining part of flue gas stream 96 is discharged as
a stream 99. Separation zone 98 contains a row of oxygen transport
membranes 100, 101, 102, 104, 106, 108 and 110 that are connected
in series by lance tubes and function in a similar manner to oxygen
transport membranes 54, 56, 58, and 60 shown in the embodiment of
furnace 2. Part 97 of flue gas stream 96 acts as a sweep gas
passing in the counter-current flow direction to the compressed air
and retentate streams to facilitate the separation of oxygen in the
same manner described with reference to furnace 2.
The sweep gas after having passed through separation zone 98
becomes oxygen enriched to form an oxygen-enriched sweep gas stream
112. Oxygen-enriched sweep gas stream 112 is circulated back to
combustion zone 90 to support combustion of the fuel by means that
include a heat exchanger 114 that acts to cool oxygen-enriched
sweep gas stream 112 and form a cooled sweep gas stream 116. Such
means also include a blower 118 that is connected between heat
exchanger 114 and an inlet 119 of combustion zone 90 to supply the
motive force for such circulation.
It is to be noted that in any embodiment of the present invention
where circulation is required, such circulation can be effected by
more direct means such as a high temperature blower. Furthermore,
although oxygen transport membranes 54, 56, 58 and 60 for furnace 2
and oxygen transport membranes 100, 101, 102, 104, 106, 108 and 110
are heated to operational temperature by the respective sweep gas
streams, other embodiments are possible. For instance, the air or
other oxygen containing feed to the oxygen transport membranes
could be heated by such means as the combustion of fuel in the
feed. Separate heaters could also be used. In such cases, all of
the sweep gas might be sufficiently cooled to be circulated by a
blower alone. Although the circulation is illustrated as being in
cross flow, embodiments of the present invention are possible in
which the oxygen transport membranes are oriented parallel to the
flow of sweep gas (at right angles to the illustrated orientation.)
For instance, the oxygen transport membranes might be in an annular
arrangement surrounding a central combustion zone producing heated
flue gas that would in part be circulated from the central
combustion zone to the oxygen transport membranes.
As may be appreciated, although a single row of oxygen transport
membranes is illustrated for each of the embodiments shown in FIGS.
1-3 (for instance, oxygen transport membranes 24, 26, 28 and 30 of
boiler 1), embodiments of the present invention are possible that
employ multiple rows of oxygen transport membranes receiving
compressed air from an inlet manifold and discharging an oxygen
lean or a nitrogen product to a discharge manifold. If more than
one row of oxygen transport membranes were used, adjacent sets of
oxygen transport membranes, as viewed transversely to the rows of
oxygen transport membranes, would be manifolded together so that
each adjacent set of oxygen transport membranes would produce
retentate streams having like oxygen concentrations that would be
combined and passed to a successive adjacent set of oxygen
transport membranes. In any embodiment of the present invention,
including those in which the oxygen transport membranes are not
connected in series, the rows of oxygen transport membranes can be
staggered so that each oxygen transport membrane has the full
benefit of the cross-flow action of the reactive purge or sweep
gas. Additionally, although the oxygen transport membranes are
illustrated as closed-end tubes, other forms of oxygen transport
membranes are possible such as plates.
Other applications of the present invention are possible beyond
those illustrated in the embodiments shown in FIGS. 1-3. For
instance, a reactor to produce a nitrogen-enriched product stream
might have a similar schematic representation to boiler 1. In such
case although such reactor might be provided with steam tubes 12,
14, 16, and 18, other tubes containing any suitable heat transfer
fluid might be employed. Similarly, a reactor having a similar
design to that shown in FIG. 2 could be used for the sole purpose
of producing a nitrogen-enriched product stream. In such
applications of the present invention, the incoming air feed might
be compressed if the product were desired at pressure.
Although not illustrated, embodiments of the present invention are
possible in which the oxygen transport membranes are not connected
in series, but enjoy the benefits of a cross-flow reactive purge or
sweep gas. Further, although the present invention has been
illustrated in connection with one or more rows of oxygen transport
membranes, an embodiment employing a single oxygen transport
membrane is possible.
As stated above, temperature control of the oxygen transport
membranes is critical, particularly in the case, such as
illustrated in FIG. 1 in which a reactive purge is produced by the
combustion of fuel at the surface of an oxygen transport membrane.
In this regard, the present invention, in addition to any method or
device that utilizes a cooled sweep gas for temperature control
(such has been discussed with reference to the embodiment shown in
FIG. 2) also encompasses any method or device in which a reactive
purge is utilized. As such, the present invention is intended to
cover a single or multiple oxygen transport membranes subjected to
a reactive purge in which temperature is controlled with an
appropriate heat sink. Such aspect of the present invention is
therefore not limited to the use of a counter-current reactive
purge. Further, the heat sink can be flowing heat transfer fluid,
water in case of a boiler, or a melt and refractory lining within a
furnace in case of a furnace.
In any configuration of oxygen transport membranes, involving a
reactive purge, the combustion of fuel will take place on or at
least near the surface of a membrane. For instance, in case of a
single membrane, fuel will combust at the leading surface of the
membrane. As the fuel flow travels around the membrane, it mixes
with combustion products and is diluted. The dilution of the fuel
decreases the driving force for the diffusion of the fuel to the
surface of the membrane and hence, produces combustion of the fuel
at or near the surface of the membrane. The same holds true for
dilution of the fuel as it passes to successive membranes in a row
of membranes.
Under conditions of combustion encountered in the present
invention, namely, combustion at or near the surface of the
membrane, heat transfer by the mechanism of radiation will
dominate. Therefore, a heat sink employed in connection with such
an oxygen transport membrane must be designed and employed to
sufficiently absorb the radiant heat that thermal runaway is
prevented. With reference to FIG. 4, a calculated example is shown
of an oxygen transport membrane of tubular form surrounded by six
steam tubes. For purposes of the example, the oxygen transport
membrane was assumed to have an oxygen flux of 20 scfh/ft.sup.2
throughout the optimum operating range. In this example it was
assumed that both the steam tubes and the oxygen transport membrane
acted as black bodies with the field of view between the oxygen
transport membranes and the surrounding steam tubes estimated by
the crossed string method. The combustion flux for the membrane was
set at 9000 BTU/ft.sup.2 and the steam tube temperature was fixed
at 400.degree. F. The upper limit of the operating range of the
membrane is that temperature at which the membrane will fail. The
lower limit is the temperature at which the membrane will cease to
function. As illustrated, the steam tubes must constitute at least
about 58% of the total surface area of the membrane and the steam
tubes to prevent the membrane from overheating. At the other
extreme, a ratio of greater than about 85% leads to excessive
cooling of the membranes.
With reference to FIG. 5, an example of an oxygen transport
membrane located within a glass furnace is illustrated. In case of
a refractory lined roof of a glass furnace, the oxygen transport
membranes will be positioned immediately adjacent to the roof. The
oxygen transport membranes will "see" for purposes of radiative
heat transfer a planar heat sink which would be the glass bath. The
roof can be assumed to be in radiative equilibrium with such planar
heat sink. Assuming such an arrangement, and, again, assuming
tubular oxygen transport membranes with all surfaces acting as
black bodies, the equilibrium surface temperature of the oxygen
transport membranes can be calculated for a given center to center
spacing of the membranes. For purposes of the example, the heat
sink was assumed to be at 1200.degree. F. and the combustion flux
was again set at 9000 BTU/hour. In this example, it can be seen
that the ratio of center to center tube spacing to tube diameter
must be greater than about 3 to avoid excessive temperatures.
Although the present invention has been described with reference to
preferred embodiments as will occur to those skilled in the art,
numerous changes, additions and omissions may be made without
departing from the spirit and the scope of the present
invention.
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